1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to a top emission type organic electroluminescent display device and a method of fabricating the same.
2. Discussion of the Related Art
Among flat panel displays, organic electroluminescent displays, have properties of high brightness and low driving voltages. In addition, because they are self-luminous, the organic electroluminescent displays have excellent contrast ratios and have ultra thin thicknesses. The organic electroluminescent displays have response time of several micro seconds, and there are advantages in displaying moving images. The organic electroluminescent displays have wide viewing angles and are stable under low temperatures. Since the organic electroluminescent displays are driven by low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits.
The organic electroluminescent displays are classified into a passive matrix type and an active matrix type. In the passive matrix type, scan lines and signal lines cross each other to form diodes, and the signal lines are sequentially scanned to drive each pixel. To obtain required average brightness, instant brightness is needed which is the product of average brightness and the number of lines.
On the other hand, in the active matrix type, a thin film transistor, as a switching element, is formed in each sub-pixel. A first electrode connected to the thin film transistor turns on/off by the sub-pixel, and a second electrode facing the first electrode functions as a common electrode. In addition, a voltage applied to the sub-pixel is stored in a storage capacitor, and the voltage is maintained until the signal of next frame is applied. Accordingly, regardless of the number of the scan lines, the sub-pixels are continuously driven during one frame. Even though low currents are applied, brightness may be constant. Therefore, recently, the active matrix type organic electroluminescent displays have widely used because of their low power consumption, high definition and large-sized possibility.
FIG. 1 is an equivalent circuit diagram illustrating a pixel of an active matrix organic electroluminescent display device according to the related art.
In FIG. 1, the pixel of the active matrix organic electroluminescent display device includes a switching thin film transistor STr, a driving thin film transistor DTr, a storage capacitor StgC, and an organic electroluminescent diode E.
More particularly, a gate line GL is formed along a first direction. A data line DL is formed along a second direction crossing the first direction and defines a pixel region P with the gate line GL. A power line PL for supplying a source voltage is spaced apart from the data line DL.
The switching thin film transistor STr is formed at a crossing portion of the gate line GL and the data line DL. The driving thin film transistor DTr is electrically connected to the switching thin film transistor STr. The organic electroluminescent diode E includes a first electrode connected to a drain electrode of the driving thin film transistor DTr and a second electrode connected to the power line PL. The power line PL supplies the source voltage to the organic electroluminescent diode E. The storage capacitor StgC is formed between a gate electrode and a source electrode of the driving thin film transistor DTr.
A scan signal is applied to the switching thin film transistor STr through the gate line GL, and the switching thin film transistor STr turns on. Then, a data signal from the data line DL is supplied to the gate electrode of the driving thin film transistor DTr, and the driving thin film transistor DTr turns on. Accordingly, the organic electroluminescent emits light. Here, when the driving thin film transistor DTr is in on-state, levels of currents flowing in the organic electroluminescent diode E from the power line PL are determined. The organic electroluminescent diode E has gray scales according to the levels of the currents. When the switching thin film transistor STr turns off, the storage capacitor StgC maintains a gate voltage of the driving thin film transistor DTr constant. Even though the switching thin film transistor STr is in off-state, the levels of the currents flowing in the organic electroluminescent diode D are constantly maintained until a next frame.
The organic electroluminescent display device is classified into a top emission type and a bottom emission type according to a direction of light emitted from the organic electroluminescent diode. The bottom emission type has a disadvantage of low aperture ratio, and recently the top emission type has been widely used.
FIG. 2 is a schematic cross-sectional view of a top emission type organic electroluminescent display device according to the related art.
In FIG. 2, first and second substrates 10 and 70 are disposed to face each other. Peripheries of the first and second substrates 10 and 70 are sealed by a seal pattern 80. A driving thin film transistor DTr is formed in each pixel region P on the first substrate 10. A passivation layer 40 is formed on the driving thin film transistor DTr and has a drain contact hole 43. A first electrode 47 is formed on the passivation layer 40 and contacts an electrode (not shown) of the driving thin film transistor DTr through the drain contact hole 43.
An organic emission layer 55 is formed on the first electrode 47 in each pixel region P. The organic emission layer 55 includes red, green and blue organic luminous patterns 55a, 55b and 55c each corresponding to the pixel region P. A second electrode 58 is formed on the organic emission layer 55 all over the surface of the first substrate 10. The first and second electrodes 47 and 58 provide electrons and holes. The first electrode 47, the organic emission layer 55 and the second electrode 58 sequentially layered constitute an organic electroluminescent diode E.
The first substrate 10 and the second substrate 70 are attached by the seal pattern 80, and the second electrode 58 on the first substrate 10 is spaced apart from the second substrate 70.
FIG. 3 is a cross-sectional view of a pixel region of a top emission type organic electroluminescent display device according to the related art. The pixel region includes a driving thin film transistor.
In FIG. 3, the driving thin film transistor DTr is formed on a first substrate 10. The driving thin film transistor DTr includes a gate electrode 13, a gate insulating layer 16, a semiconductor layer 20 including an active layer 20a and ohmic contact layers 20b, and source and drain electrodes 33 and 36 sequentially layered on the first substrate 10. The active layer 20a is formed of intrinsic amorphous silicon. The ohmic contact layers 20b are formed of impurity-doped amorphous silicon and are spaced apart from each other on the active layer 20a. The source and drain electrodes 33 and 36 are spaced apart from each other and are connected to a power line (not shown) and an organic electroluminescent diode E, respectively.
The organic electroluminescent diode E includes first and second electrodes facing each other and an organic emission layer 55 interposed therebetween. The first electrode 47 is formed in each pixel region P and contacts an electrode of the driving thin film transistor DTr. The second electrode 58 is formed on the organic emission layer 55 all over the surface of the first substrate 10.
A second substrate 70 for encapsulation is disposed over and faces the first substrate 10 including the above-mentioned elements, and the first and second substrates 70 form an organic electroluminescent display device 1.
In the top emission type organic electroluminescent display device 1, when the driving thin film transistor DTr is a p-type, the first electrode 47 is formed of a transparent conductive material having relatively high work function, such as indium tin oxide or indium zinc oxide, so as to function as an anode electrode, and the second electrode 58 is formed of a metallic material having relatively low work function so as to function as a cathode electrode.
However, the metallic material, which is used for the second electrode 58 functioning as the cathode electrode and has relatively low work function, is opaque. Therefore, if the opaque metallic material is deposited to have a thickness of an ordinary electrode or insulating layer, that is, several thousand angstroms (Å), light cannot pass through the second electrode 58, and the top emission cannot be achieved.
To keep its transparency, the second electrode 58, which is formed of an opaque metallic material having relatively low work function, may have a double-layered structure including a lower layer (not shown) of an opaque metallic material and an upper layer (not shown) of a transparent conductive material, wherein the lower layer has a thickness of several ten angstroms (Å) to several hundred angstroms (Å), and the upper layer has a thickness of several thousand angstroms (Å). With respect to the first electrode 47, which is formed of a transparent conductive material having relatively high work function and functions as an anode electrode, a reflective layer (not shown) of a material having relatively high reflectivity is further formed under the first electrode 47 to reflect light and increase emission efficiency.
However, the transparent conductive material is generally deposited by a sputtering method. The sputtering method has a mechanism in which atoms or molecules are ejected from a target due to collision with particles having high energy and are adsorbed to a surface of a substrate. Accordingly, the atoms or particles have high energy and thus damage the surface of the substrate or a treated material layer. Particularly, since an organic insulating layer is formed by a thermal deposition method and has a relatively weak surface, it is impossible to form a transparent conductive layer on the organic insulating layer by the sputtering method. In addition, when a transparent conductive layer is formed on a metallic layer, which is formed of a thermal deposition method, by the sputtering method, the metallic layer may be transformed due to surface damage or may have poor functions because the particles from the target penetrate into the metallic layer and lower the properties of the metallic layer.
To solve the problem, an electron beam deposition method has been suggested as a method for depositing a transparent conductive material. In the electron beam deposition method, an electron beam, which is generated from a thermal ion electron beam gun or a plasma electron beam gun, is irradiated to a target, and the target is partly heated and evaporated to thereby form a layer, which is made of a material for the target, on a surface of a substrate. Accordingly, there is no damage on the surface of the substrate, and even though a film has a weak surface, a predetermined material layer can be formed on the film by the electron beam deposition method without damage.
By the way, the electron beam method causes another problem. When the electron beam is generated or the electro beam is irradiated to the target, X-ray is generated. The X-ray goes into the driving and switching thin film transistors under the first electrode and decreases characteristics of the thin film transistors. Especially, when the X-ray is incident on a channel of a thin film transistor, off currents of the thin film transistor rapidly increase, and a threshold voltage increases. Therefore, functions of the thin film transistor are remarkably lowered.
Accordingly, to form a film by the electron beam deposition method, a shield layer for blocking the X-ray is needed over the driving and switching thin film transistors. The shield layer is formed of a metallic material having an atomic density of about 10 g/cm3 to about 30 g/cm3, for example, tungsten or lead. Here, since lead has a relatively very low melting point, lead can be melted during a thermal deposition process, and tungsten may be used as an x-ray shield layer.
However, tungsten has been seldom used in the organic electroluminescent display device, and it has not been considered that etchant for tungsten affects elements of the organic electroluminescent display device. According to this, when etching tungsten, other elements may be removed together. Moreover, etching bath and rinse equipment for patterning tungsten are required, and this causes an increase in initial equipment costs.